The ability to directly convert methane to methanol in economically satisfactory yields is an important goal of the oil and gas industry. Methane is an abundant material found world-wide, particularly in the form of natural gas. As a gas, it is difficult and costly to transport. Conversion to the liquid methanol allows for safer, more efficient transportation. In addition, methanol is a valuable commercial feedstock, an important ingredient in the production of reformulated motor fuels, and an environmentally compatible fuel in itself.
The conventional method for the catalytic conversion of methane to methanol involves a first reaction with water (steam reforming) to produce synthesis gas, which is a mixture of carbon monoxide and hydrogen, followed by catalytic conversion of the synthesis gas to methanol. A direct, one-step oxidation of methane to methanol would be simpler, and economically and environmentally preferable.
Several catalytic and non-catalytic approaches to directly converting methane to methanol are known in the art. Among these are the following catalytic processes:
United Kingdom Pat. No. 1,244,001 discloses the oxidation of methane to methanol over a catalyst consisting of (Mo.sub.2 O.sub.3).sup.. Fe.sub.2 O.sub.3 on silica/alumina (25% Al.sub.2 O.sub.3 /75% SiO.sub.2), sintered to 0.1 g/cm.sup.2 at 1000.degree. C., with 65% selectivity (moles methanol/moles product .times.100) at 2.1% conversion. The temperature disclosed is 439.degree. C. and the pressure 52 atmospheres. Temperatures, pressures and space rates in the process disclosed in this patent are 300.degree.-550.degree. C.; 5-150 atmospheres; and 20,000-50,000 hr.sup.-1, respectively.
Eusuf, Sci. Res., Dacca (1969) Vol VI, Nos. 1,2, p.16, discloses the oxidation of methane to methanol over CrO.sub.3 /pumice. The reported results indicated 12% selectivity at 11% O.sub.2 conversion. The reported 8.9% methane conversion is noted to most likely be an error as indicated by the reported carbon/oxygen balance. The actual conversion rate may have been far lower.
Further results on the chlorine-promoted oxidation of methane to methanol over CrO.sub.3 /pumice were reported in Eusuf, Bangl. J. Sci. Ind. Res. (1975) Vol. 10, Nos 1-2, pp. 135-141 ("Eusuf II"). Eusuf II discloses methane conversion as high as 7.3%, with yields of methanol on input methane basis as high as 46.4%. These results were observed at a temperature of 430.degree. C., pressure at 1.5 atmospheres, and a contact time of 1.5 seconds. The reaction was run in the presence of Cl.sub.2 at a volumetric ratio of 0.10, Cl.sub.2 :CH.sub.4, indicating that there was more chlorine gas present than the amount of methane converted in the reaction.
Few, if any, catalysts currently exist, however, which will promote the direct oxidation of methane to methanol in commercially acceptable yield and selectivity. Durante et al, U.S. Pat. No. 4,918,249, assigned to Sun Company, Inc. (R&M), disclose oxidation of methane to methanol in 70% selectivity at 90% oxygen conversion over an iron framework substituted sodalite catalyst at temperatures around 415.degree. C.
Han et al., U.S. Pat. No. 5,015,798, disclose methane conversion over aluminosilicate zeolite catalyst. Using ZSM-5 zeolite as catalyst, Han et al. disclose methane oxidation to methanol with 5.2% methane conversion and 16.7% methanol selectivity at 450.degree. C. and 960 psig.
Most catalysts which contain oxidation-active transition metals do not produce significant amounts of methanol as oxidation product, but rather tend to combust methane to give carbon oxides and water at the elevated temperatures necessary for oxidation to occur. A catalyst which can oxidize methane to methanol at low temperatures could be very important in producing better selectivities by reducing unwanted carbon oxides.
The process of the present invention involves direct air or oxygen conversion of methane to methanol. No promoter, such as chlorine gas, need be present, which has the added advantage of avoiding the production of chlorocarbon compounds and the creation of a highly corrosive chlorine-containing reaction system.